Electronic systems, including, for example, laptop, hand-held and desktop computers as well as cell phones operate through the use of input electrical power. These have the characteristic that some of the input power is converted to heat, and the heat generated is typically concentrated in an identifiable area, such as an integrated circuit chip or a circuit board. Such devices and products incorporate heat sinks, fans, heat-pipes, refrigeration, and cooling water as needed to regulate their operating temperature within specific ranges.
It is predicted that electronic devices currently under development will generate heat at total rates and spatial densities exceeding the cooling capabilities of conventional heat sinking technology. For example, technology targets set by the semiconductor industry and summarized by the International Technology Roadmap for Semiconductors indicate the need to remove as much as 200 W from the surface of a microprocessor before the year 2007. The trends towards higher total power and power density are similarly extreme in the optical telecommunications industry, the power electronics industry, and elsewhere. Conventional heat sinks are simply incapable of removing the targeted powers and power densities within a volume consistent with system design and market expectations.
This situation is exacerbated by targeted reductions of total system volume, which diminish the volume and surface area available for cooling devices. The interaction of these two trends (increasing head load and decreasing system volume) are recognized as a critical problem for the future of the semiconductor industry, and for other industries that rely on heat-generating or absorbing devices.
Conventional techniques for removing heat from devices include a number of well-established technologies.
Heat sinks generally consist of metal plates with fins that transport heat from the device to the surrounding air by natural or forced convection. The heat sink fins serve to increase the area of contact between the device and the air, thereby increasing the efficiency of the heat transfer. Heat sinks of many materials, geometries, and constructions have been known for more than 50 years.
Fans consisting of rotating blades driven by electric motors can enhance the heat transfer between a heat sink and the surrounding air by causing the air to circulate around and through the heat sink with greater velocity than that which results from natural convection. Fans have been used for cooling systems for more than 30 years. Integration of fans with heat sinks for cooling of devices which generate large quantities of heat have been developed by many inventors, and are in wide use.
Heat pipes consist of a hollow tube which incorporates a wicking structure, and is partially filled with liquid. One end of the heat pipe is placed in contact with the heat-generating device. At this end of the heat pipe, the liquid evaporates, and vapor travels down the hollow center of the pipe to the other end. This end is placed into contact with a cold medium, or a heat sink, or is in contact with the surrounding air, and acts to cool the vapor in the center of the tube to the condensation temperature. This liquid, after condensation, is transported back to the hot end of the tube by capillary forces within the wicking structure. Heat pipes can offer significantly better heat conduction than solid metal rods of the same dimensions, and are widely used in many applications. Heat pipes are presently used for heat removal in electronic products, spacecraft, and a variety of other applications where heat generation in compact geometries is of interest. Heat pipes may be formed in many geometric structures, and may be integrated into the device package for efficient heat transport (U.S. Pat. No. 5,216,580), or may be used to deliver heat some distance away through a flexible coupling (U.S. Pat. No. 5,560,423). Vapor chambers closely resembles a heat pipe in operating principle and dimensions, but generally has a rectangular (rather than tubular) cross-sectional shape and can involve varying geometrical placements of the wicking structure. Both heat pipes and vapor chambers are subject to the same basic physical limit for the peak power: The longer the separation between the heat source and the heat sink, and the smaller the cross sectional area, the smaller the total heat power that can be removed before these devices reach the capillary limit, or dry-out, condition.
Active cooling of a device via a vapor-compression cycle or by thermoelectric or other solid-state cooling devices is used in some high-performance thermal control applications, or where regulation at low temperatures is required. In these applications, the heat from the device is transported to the rejection surface, and significant excess heat is added due to the limited thermodynamic efficiency of the cooling mechanism. The rejection surface must generally be cooled by one of the other means described herein. Vapor-compression refrigeration cycles have been used in many applications ranging from home air-conditioning and kitchen refrigeration to spacecraft and cryogenic systems for many years.
Cooling water is used in situations where large quantities of heat are generated, and the other methods described herein are unable to reject the heat to the surrounding air. In this case, a continuous supply of cool water is required, and this cool water is passed around or through the device or channels in an attached structure. Thereafter, the warmer water is returned to the waste water system.
In miniaturized applications employing cooling water, techniques exist which address the problem of miniaturized microchannel cooling systems, miniaturized, closed-loop cooling systems, and systems which rely on active pumping of fluids to achieve cooling. Microchannel heat exchangers were originally explored by Tuckerman et al. (U.S. Pat. Nos. 4,450,472, 4,573,067) in the early 1980s. These devices contained straight, uniform-cross-section microfabricated channels within a silicon substrate, through which liquid coolant was passed. Subsequent patents followed the original work of Tuckerman and Pease (U.S. Pat. No. 4,450,472), including descriptions of microchannel fabrication methods, attachment methods, and specific materials and designs for specific applications.
Microchannel heat sink design to achieve higher heat transfer coefficients or improved temperature uniformity has also been explored. Phillips (U.S. Pat. No. 4,894,709) described liquid microchannel cooling with a guard header structure to improve temperature uniformity in the chip. Frieser (U.S. Pat. No. 4,312,012) described modifications of the surface of the microchannel to improve nucleate boiling and the heat transfer coefficient. Swift (U.S. Pat. No. 4,516,632) and Walpole (U.S. Pat. No. 5,099,910) described channels with alternating flow directions to improve temperature uniformity. Lomolino (U.S. Pat. No. 5,427,174) used a two-fluid mixture to control the effective heat capacity of the coolant and turbulence over a targeted temperature range.
Closed-loop cooling systems employing microchannels have also been an active area of research in recent years, including the description by Hamilton (U.S. Pat. No. 5,901,037) of a closed-loop cooling system in which fluids are passed through microchannels attached to the integrated circuit, and a magnetic pump generates the pressure. Further, Davis (U.S. Pat. No. 5,703,536) describes the use of a closed loop fluidic cooling system for cooling of high-power RF transmitters.
There has been extensive research into the development of micropumps. These research efforts include pumps based on oscillating piezoelectric membranes, peristaltic pumps, electrohydrodynamic pumps, and others. These pumps, to date, appear to be incapable of generating the pressure and/or flow necessary for application to removal of high heat flux from high-power devices.
The phenomenon of electro-osmosis has been known since the work of F. F. Reuss in 1809. A simple description of this phenomenon is that liquid flow is induced on a region of net charge that develops at the liquid/wall interface. The magnitude of the force is proportional to the applied electric field, and the quantity of the charged species available in this region of net charge. Larger flow rates can be achieved for systems with large cross-sectional areas. Large pressure generation requires structures with very high surface-to-volume ratio.
Miniature pumps based on the phenomenon of electro-osmosis (i.e., electroosmotic pumps) were originally developed by Theeuwes (U.S. Pat. No. 3,923,426), in which a porous ceramic structure was used to provide a multitude of micron-sized pathways with charged surface layers. Theeuwes describes the importance of selecting pumping structures which feature high porosity, high electroosmotic mobility for a given working fluid, small diameter pores, and discusses the possibility of the use of quartz or glass ceramics, possibly comprised of beads, and porous polymer matrices. The working fluid in the Theeuwes pump was suggested to have a high dielectric constant, low viscosity, and low electrical conductivity. Example liquids that the Theeuwes pump used include deionized water, ethyl-alcohol and alcohol-water mixtures, and many organic solutions. With these materials and solutions, flow rates in excess of 1 mL/min and pressures exceeding 1 Atmosphere were reported.
Despite the many different and diverse cooling systems and techniques described above, there exists a need for an improved closed-loop, fluidic cooling system and techniques for high power applications having the capability of being implemented in a miniaturized environment. In addition, there exists a need for an improved miniature fluidic cooling system having feedback-controlled temperature regulation of devices that facilitates, for example, hotspot cooling by way of, for example, active regulation of the temperature of the integrated circuit device through electrical control of the flow through the pump. Such miniature fluidic cooling system may utilize multiple cooling loops (in conjunction with multiple pumps) to allow independent regulation of the special and temporal characteristics of the device temperature profiles.
Moreover, there exists a need for a miniature pump that is capable of generating the high pressure (for example, pressure greater than 10 PSI) and/or high flow (for example, a flow rate greater than 5 ml/min) that are necessary for the removal of the predicted high heat flux (for example, power greater than 100W). Such a pump should overcome or address the shortcomings of the conventional pumps, for example a pump configuration for use in a closed-loop system that addresses practical issues involving evolved gases (for example, by way of recapture) or deposited materials. These issues tend to be prominent in closed-loop fluidic cooling systems employing pumps.